Antifluorite

Fluorite
Deep green isolated fluorite crystal showing cubic and octahedral faces, set upon a micaceous matrix, from Erongo Mountain, Erongo Region, Namibia (overall size: 50 mm x 27 mm, crystal size: 19 mm wide, 30 g)
General
Category	Halide mineral
Formula (repeating unit)	CaF2
Strunz classification	3.AB.25
Crystal system	Isometric
Crystal class	Hexoctahedral (m3m) H–M symbol: (4/m 3 2/m) (cF12)
Space group	Fm3m (No. 225)
Unit cell	a = 5.4626 Å; Z = 4
Identification
Color	Colorless, although samples are often deeply colored owing to impurities.
Crystal habit	Well-formed coarse sized crystals; also nodular, botryoidal, rarely columnar or fibrous; granular, massive
Twinning	Common on {111}, interpenetrant, flattened
Cleavage	Octahedral, perfect on {111}, parting on {011}
Fracture	Subconchoidal to uneven
Tenacity	Brittle
Mohs scale hardness	4 (defining mineral)
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.175–3.184; to 3.56 if high in rare-earth elements
Optical properties	Isotropic; weak anomalous anisotropism
Refractive index	1.433–1.448
Fusibility	3
Solubility	slightly water soluble and in hot hydrochloric acid
Other characteristics	May be fluorescent, phosphorescent, thermoluminescencent, and/or triboluminescent
References

Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF₂. It belongs to the halide minerals. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon.

Mohs scale of mineral hardness, based on scratch Hardness comparison, defines value 4 as Fluorite.

Fluorite is a colorful mineral, both in visible and ultraviolet light, and the stone has ornamental and lapidary uses. Industrially, fluorite is used as a flux for smelting, and in the production of certain glasses and enamels. The purest grades of fluorite are a source of fluoride for hydrofluoric acid manufacture, which is the intermediate source of most fluorine-containing fine chemicals. Optically clear transparent fluorite lenses have low dispersion, so lenses made from it exhibit less chromatic aberration, making them valuable in microscopes and telescopes. Fluorite optics are also usable in the far-ultraviolet range, where conventional glasses are too absorbent for use.

The word fluorite is derived from the Latin verb fluere, meaning to flow. The mineral is used as a flux in iron smelting to decrease the viscosity of slags. The term flux comes from the Latin adjective fluxus, meaning flowing, loose, slack. The mineral fluorite was originally termed fluorospar and was first discussed in print in a 1530 work Bermannus, sive de re metallica dialogus [Bermannus; or a dialogue about the nature of metals], by Georgius Agricola, as a mineral noted for its usefulness as a flux. Agricola, a German scientist with expertise in philology, mining, and metallurgy, named fluorspar as a neo-Latinization of the German Flussspat from Fluß (stream, river) and "Spat" (meaning a nonmetallic mineral akin to gypsum, spærstān, spear stone, referring to its crystalline projections).